MOUNT TEREILL AND MOUNT MARVINE, 267 



truth, it seems little doubtful that the Fish Lake Plateau is a great center 

 of eruption. A general fact in support of this view is that in three direc- 

 tions north, south, and east— and in all intermediate directions, the mass 



of erupted material attenuates gradually. Whether this be true also of the 

 west side it is impossible to say, because the great monocline carries every- 

 thing down beneath the alluvium of Grass Valley. But in the other direc- 

 tions we can form a fair notion of the general arrangement of the total 

 extravasation, and the attenuation and radiation from a central locality is 

 sufficiently clear. The most probable view of the original arrangement is 

 that the lavas emanated from many orifices and fissures scattered over the 

 surface of an extensive volcanic pile, not unhke that of Mauna Loa, but on 



a smaller scale. 



From the outlet of Fish Lake, at its northeastern end, we may pursue 

 our way down the noble valley which carries the effluent stream. About 

 4 miles from the outlet we again enter Summit Valley, and, turning north- 

 ward, we may ascend it to the first camping-ground from which we started 

 to ascend the plateau. On the trail thither we pass two great terminal 

 moraines projecting from the openings of gorges cut back into the plateau 

 mass. Like the one projecting into the lake, they are well preserved and 

 quite typical in their features. 



MOUNT TEERILL AND MOUNT MARVINE. 



Upon the eastern side of Summit Valley rise two conspicuous masses, 

 which present to the eye nothing suggestive of a plateau. The northern 

 one is Mount TerriU, the southern is Mount Marvine, both being in the 

 prolongation of the same axis. Although in external form they are great 

 mountain piles, their origin is 'due to circumdenudation, just as a great 

 butte owes its individuality to the removal of the strata around it. They 

 consist of lavas, resting upon Lower Tertiary calcareous beds, and both the 

 lavas and the sediments are nearly horizontal so far as stratification is con- 

 cerned ; but the lavas were obviously outpoured over a much eroded sur- 

 face with hills and valleys of some magnitude. The volcanic sheets may 

 have been continuous with those of Fish Lake Plateau., since they have the 

 same lithological characters and varieties as the more striking trachytic 



